Phosphorus NMR and Ab Initio Modelling of P–N Bond Rotamers of a Sterically Crowded Chiral β‐P4S3 Diamide

Reaction of bicyclic β‐P₄S₃I₂ with enantiomerically pure (R)‐Hpthiq (1‐phenyl‐1,2,3,4‐tetrahydroisoquinoline) and Et₃N gave a solution of a single diastereomer of the unusually stable diamide β‐P₄S₃(pthiq)₂, accounting for 83 % of the phosphorus content. Despite the steric bulk of the substituents, each amide group of this could adopt either of two rotameric positions about their P–N bonds, so that, at 183 K, ³¹P NMR multiplets for four rotamers could be observed and the spectra of three of them analysed fully. The large ²J(P–P–P) coupling became greater (253, 292, 304 Hz) with decreasing abundance of the individual rotamers. The rotamers were modelled at the ab initio RHF/3–21G* level, and relative NMR chemical shifts predicted by the GIAO method using a locally dense basis set, allowing the observed spectra to be assigned to structures. Calculations at the same level for the model compound α‐P₄S₃(pthiq)Cl confirmed the assignments of low‐temperature rotamers of α‐P₄S₃(pthiq)I reported previously. Changes in observed P–P coupling constants and ³¹P chemical shifts, on rotating a pthiq substituent, could then be compared between β‐P₄S₃(pthiq)₂ and α‐P₄S₃(pthiq)I, confirming both sets of assignments. The most abundant rotamer of β‐P₄S₃(pthiq)₂ was not the one with the least sterically crowded sides of both pthiq substituents pointing towards the P₄S₃ cage, because of interaction between the two substituents. Only by using a DFT method could relative abundances of rotamers of β‐P₄S₃(pthiq)₂ be predicted to be in the observed order. Use of racemic Hpthiq gave also the two diastereomers of β‐P₄S₃(pthiq)₂ with C_s symmetry, for which the room temperature ³¹P{¹H} NMR spectra were analysed fully.